In IEEE (Institute of Electrical and Electronic Engineers) 802.11 which is a standard of wireless LAN (Local Area Network) and IEEE802.11e which is expected to become widely used from now on, and is further a standard for securing QoS (Quality of Services), a frame transmission/reception method of a communication terminal for which power saving is carried out is defined.
Hereinafter, a related communcation system 200 of FIG. 24 will be describe.
FIG. 24 is a block diagram which shows the related communication system 200. FIG. 25 is a block diagram which shows a physical layer frame of IEEE802.11. FIGS. 26 and 27 are time charts which show operations of the related communication system 200.
In FIG. 24, the related communication system 200 has a management terminal 201, and communication terminals 202a, 202b, . . . . The management terminal 201 is connected to wired LAN (first network) such as Internet, through a LAN cable and a coaxial cable which are not shown in the figure. In addition, the management terminal 201 is connected to wireless LAN (second network) through an invisible wireless communication path 203. That is, the management terminal 201 is connected to the communication terminals 202a, 202b, . . . respectively, through the wireless communication path 203, and manages transmission/reception of a frame to/from the communication terminals 202a, 202b, . . . through the wireless LAN.
In FIG. 25, 204 designates a physical layer frame of IEEE802.11 which is transmitted to the air. 205 designates a PHY (Physical Layer) header which is added in a physical layer. The PHY header 205 shows a data rate and a data link layer frame length 206 designates a data link layer frame of IEEE802.11. 207 designates a MAC (Medium Access Control) header, and the MAC header 207 shows what kind of frame the frame is, whether power saving is carried out, a destination address, a source address, etc. 208 designates a frame body. The frame body 208 stores various data depending on a content of the MAC header 207. 209 designates FCS (Frame Check Sequence), and it shows 32 bit CRC (Cyclic Redundancy Code) which was calculated from the MAC header 207 and the frame body 208, in order to confirm reasonability of contents of the MAC header 207 and the frame body 208.
Firstly, in case that the communication terminals 202a, 202b, which are in a power saving status on the basis of IEEE802.11 standard, receive data from the management terminal 201, an operation of the related communication system 200 having only a competition period for obtaining a transmission right (i.e., having no centralized management period which will be described later) will be described along FIG. 26. FIG. 26 is a time chart which shows an operation of the related communication system 200 having only the competition period.
In FIG. 26, when the management terminal 201 wishes to transmit data to the communication terminals 202a, 202b, it is not known whether the communication terminals 202a, 202b stay awake from the power saving status, and therefore, it is not possible to transmit a frame from the management terminal 201 at arbitrary timing. Then, at a time point t11, the management terminal 201 transmits a beacon frame 63 which shows that there is also data which is desired to be transmitted to the communication terminals 202a, 202b. The communication terminals 202a, 202b know that there exists data addressed to themselves in the management terminal 201, respectively, by receiving all beacon frames 63.
In order to request data to the management terminal 201, there is such a necessity that the communication terminal 202a transmits a PS (Power Save)-POLL frame 64a to the management terminal 201 and the communication terminal 202b transmits a PS-POLL frame 64b to the management terminal 201, and, in FIG. 26, there is only a competition period, and therefore, the communication terminals 202a, 202b are to compete with each other.
For example, assuming that the communication terminal 202a wins this competition, the communication terminal 202a transmits the PS-POLL frame 64a to the management terminal 201 at a time point t12. The management terminal 201, when it receives the PS-POLL frame 64a, transmits, at a time point t13, a data frame 65a which shows that data addressed to the communication terminal 202a is only this frame by the MAC header 207, having a short frame interval called as SIFS (Short Interframe Space), to the communication terminal 202a. 
The communication terminal 202a, when it receives the data frame 65a, transmits, at a time point t14, a fixed length ACK frame 66a which shows that it received the data frame 65a, having a frame interval of SIFS, to the management terminal 201. As already described above, the data frame 65a shows that data addressed to the communication terminal 202a is only this frame, and therefore, in case that there is no data in the management terminal 201 and in addition, there is no data addressed to the management terminal 201 even in the communication terminal 202a, the communication terminal 202a turns off power until a time point t17 where it turns on power, in order to be able to receive a next beacon frame 63 (time point t18). Therefore, a power-on period of the communication terminal 202a in this case turns out to be as shown by 67a (broken line), from a time point t10 until a time point t15.
On the one end, the communication terminal 202b, which lost the competition right after a first beacon frame 63, continues to be in a power-on status after that, and is competing, and continues to be beaten by the data frame 65a and the ACK frame 66a, and obtains a right of transmission for the first time, after completion of the ACK frame 66a at a time point t14, and transmits the PS-POLL frame 64b at a time point t15.
In case that the management terminal 201 has also only one of data addressed to the communication terminal 202b, the same processing as already described above will be carried out after that. Further, in case that there is no data addressed to the management terminal 201 in the communication terminal 202b, in the same manner as in the communication terminal 202a, it turns off power until the time point t17 where it turns on power, in order to be able to receive the next beacon frame 63 (time point t18). Therefore, a power-on period of the communication terminal 202b in this case turns out to be as shown by 67b (broken line), from the time point t10 until the time point t15.
Then, in case that the communication terminals 202a, 202b, which are in a power saving status on the basis of the IEEE802.11 standard, receive data from the management terminal 201 like a voice call, and after that, transmit data to the management terminal 201, an operation of the related communication system 200 having a centralized management period and a competition period will be described along FIG. 27. FIG. 27 is a time chart which shows an operation of the related communication system 200 having a centralized management period and a competition period.
In FIG. 27, By using IEEE802.11e, QoS is secured and audio data is transferred. When the management terminal 201 wishes to transmit data to the communication terminals 202a, 202b, it is known that the communication terminals 202a, 202b stay awake during a period of a centralized management period although the communication terminals 202a, 202b are in a power saving status, and therefore, it is possible to transmit a frame from the management terminal 201 at arbitrary timing.
Inversely, when the communication terminals 202a, 202b wish to transmit data to the management terminal 201 during the centralized management period, it is not possible to transmit data unless the management terminal 201 gives permission. In addition, the management terminal 201, in case that it terminates the centralized management period earlier than a period which was notified in the beacon frame 63, has to notify the termination of the centralized management period to the communication terminals 202a, 202b. 
Firstly, at a time point t21, the management terminal 201 transmits the beacon frame 63 which shows that there is also data which is desired to be transmitted to the communication terminals 202a, 202b. The communication terminals 202a, 202b know that there exists data addressed to themselves in the management terminal 201, respectively, and they enter into a centralized management period, since they are to receive all beacon frames 63. In the centralized management period, as described above, the communication terminals 202a, 202b continue to be awake, and the management terminal 201 transmit, at a time point t22, a QoS data+CF-Poll frame 69a which carries out transmission of data addressed to the communication terminal 202a and permission of data from the communication terminal 202a at the same time, to the communication terminal 202a. 
When the communication terminal 202a, in case that it has data addressed to the management terminal 201, receives the QoS data+CF-Poll frame 69a, it transmits, at a time point t23, a QoS data+CF-Ack frame 70a which carries out showing that it received the QoS data+CF-Poll frame 69a during the centralized management period and data transmission to the management terminal 201, at the same time, to the management terminal 201.
The management terminal 201, when it receives the QoS data+CF-Ack frame 70a, transmits a QoS CF-Ack frame 71 which shows that it received data from the communication terminal 202a during the centralized management period, to the communication terminal 202a. The communication terminal 202a, when it receives the QoS CF-Ack frame 71, notifies to the management terminal 201 such a thing that there exist no other data in the management terminal 201, by a content of the MAC header 207 of the QoS data+CF-Poll frame 69a, and notifies to the management terminal 201 such a thing that there is no other data even in itself, by the QoS data+CF-Ack frame 70a, and therefore, it turns off power until a time point t28 where it turns on power, in order to be able to receive a next beacon frame 63 (time point t29). Therefore, power-on time of the communication terminal 202a in this case turns out to be as shown by 67a (broken line), from a time point t20 until a time point t25.
On the one hand, the communication terminal 202b continues to be turned power-on for quite a while since there is no instruction from the management terminal 201, and at the time point t25, the management terminal 201 transmits a QoS data+CF-Poll frame 69b, to the communication terminal 202b. 
By the received the QoS data+CF-Ack frame 69b, processing, which was already described, is carried out, and at a time point t26, the management terminal 201 transmits a CF-End+CF-Ack frame 72, in lieu of the QoS CF-Ack frame 71. This is because of notifying to terminate the centralized management period, since there is no data in both of the management terminal 201 and the communication terminals 202a, 202b. 
At a time point t27, the communication terminal 202b turns off power until a time point t28 where it turns on power, in order to be able to receive a next beacon frame 63 (time point t29), in the same manner as in the communication terminal 202a. Power-on time of the communication terminal 202b in this case turns out to be as shown by 67b (broken line), from the time point t20 until the time point t27.
These specifications are shown in the IEEE802.11 standard (e.g., see, “ANSI/IEEE Standard 802.11”, 1999, P. 128-133).
However, in the above-described communication system, a communication terminals, which terminated transmission/reception of data earlier, enters into a power saving status, but a communication terminal, which has not yet done transmission/reception of data, has to keep awake, even during such a period that another communication terminal is transmitting/receiving, and therefore, electric power is consumed wastefully, and especially, in case that a communication terminal carries out a voice call, there was such a problem that duration of call shortens.
The invention aims, in view of the above-described problems, to provide a communication system, a communication terminal, and a communication method, which can suppress power consumption of a communication terminal, by turning off power of a communication terminal which does not carry out transmission/reception, during such a period that a management terminal is transmitting to/receiving from another communication terminal.